1. Field of the Invention
The present invention relates to a digital image forming apparatus capable of converting input image data into a format which can not be recognized (hereinafter referred as a cryptograph) and preserving the converted cryptograph image data, and capable of restoring the cryptograph image data.
2. Description of the Related Art
Conventionally, confidential documents are handled by putting stamps on the confidential documents marking them as, e.g., "CONFIDENTIAL".
In the conventional method, it is difficult to keep confidential documents secret, as long as the documents are in a usual form which can be recognized by persons even if such stamps which mean a secret are displayed on the documents.